A significant fraction of Swahili vocabulary derives from Arabic, in part conveyed by Arabic-speaking Muslim inhabitants. For example, the Swahili word for "book" is traceable back to the Arabic word كتاب kitāb. However, the Swahili plural form of this word is vitabu, rather than the Arabic plural form كتب kutub, following the Bantu grammar in which ki- is reanalysed as a nominal class prefix, whose plural is vi-. Swahili is a Bantu language of the Sabaki branch. In Guthrie's geographic classification, Swahili is in Bantu zone G, whereas the other Sabaki languages are in zone E70 under the name Nyika. Local folk-theories of the language have considered Swahili to be a mixed language because of its many loan words from Arabic, the fact that the Swahili language emerged as a result of trade between the east African coastal Bantu speaking tribes and traders from Arabia, Asia as well as Europe. However, historical linguists do not consider the Arabic influence on Swahili to be significant enough to classify it as a mixed language, since Arabic influence is limited to lexical items, most of which have only been borrowed after 1500, while the grammatical and syntactic structure of the language is Bantu.

The Swahili language emerged as a result of trade between the east African coastal Bantu speaking tribes and traders from Arabia, Asia as well as Europe. The swahili language or people didn't exist prior to the 1700s as a distinct entity and its precursor language is the Pokomo language; the Swahili language is a daughter language of the Pokomo language, known as Kingozi. Most of the Bantu Swahili vocabulary is derived from the Pokomo and Mijikenda languages and secondarily from other East African Bantu languages. 30% of the Swahili vocabulary is derived from Arabic, Hindustani and Malay with Arabic contributing a majority of the foreign loan words in the Swahili language. It was written in Arabic script; the earliest known documents written in Swahili are letters written in Kilwa in 1711 in the Arabic script that were sent to the Portuguese of Mozambique and their local allies. The original letters are preserved in the Historical Archives of India, its name comes from Arabic: سَاحِل sāħil = "coast", broken plural سَوَاحِل sawāħil = "coasts", سَوَاحِلِىّ sawāħilï = "of coasts".

Since Swahili was the language of commerce in East Africa, the colonial administrators wanted to standardize it. In June 1928, an interterritorial conference attended by representatives of Kenya, Tanganyika and Zanzibar took place in Mombasa; the Zanzibar dialect was chosen as standard Swahili for those areas, the standard orthography for Swahili was adopted. Swahili has become a second language spoken by tens of millions in three African Great Lakes countries where it is an official or national language. In 1985, with the 8–4–4 system of education, Swahili was made a compulsory subject in all Kenyan schools. Swahili and related languages are spoken by small numbers of people in Burundi, Malawi, Uganda and Rwanda; the language was still understood in the southern ports of the Red Sea in the 20th century. Some 80 percent of 62 million Tanzanians speak Swahili in addition to their first languages; the five eastern provinces of the DRC are Swahili-speaking. Nearly half the 81 million Congolese speak it.

Swahili speakers may number 120 to 150 million in total. Swahili is among the first languages in Africa for which language technology applications have been developed. Arvi Hurskainen is one of the early developers; the applications include a spelling checker, part-of-speech tagging, a language learning software, an analysed Swahili text corpus of 25 million words, an electronic dictionary, machine translation between Swahili and English. The development of language technology strengthens the position of Swahili as a modern medium of communication. Swahili in East Africa has over time gained influence over; as some scholars have observed, there have been ways in which Swahili's involvement in the development of nationhood in a Swahili-speaking country like Kenya has had an influence over how the people view themselves. For instance, in Kenya, the multiplicity of ethnic tribal communities and identities have, through Swahili, found ways of seeing themselves as a bod